Again, the day of the week mismatch (weekends the past 2 years, a weekday this year) make it tricky to draw any conclusions, but it sounds like things are off to a good start. In addition, bear in mind that every year there are more people who are currently enrolleed, most of whom presumably don't need to resubmit an application (I'm a little uncertain about this, however; it may also include current enrollees who have to update their data and re-submit new applications).

UPDATE: OK, I've confirmed that yes, updating your existing application with new data (change in income/dependents/etc) does indeed count as an "application submission", so it's not quite as significant as it may sound. Still a solid launch, however.